This invention relates to a method of reproportionating and restoring desirable properties to a hydride-forming material after a degeneration of these properties has occurred from repeated absorption/desorption cycles. In particular, this invention relates to a method of substantially restoring the original pressure-composition characteristics of hydrides after repeated cycles have caused disproportionation of the material. More particularly, this invention relates to a method of substantially reversing the disproportionation process in hydrides to obtain increased capacity and approximately isobaric behavior over a wide absorption range. This invention also relates to methods of in-situ regeneration of disproportionated hydrides.
Hydrides have the ability to chemically store large quantities of hydrogen in a concentrated form at a variety of temperatures and pressures, and then to release the stored hydrogen at a higher temperature. Numerous hydride-forming materials have been identified, and various hydrides, especially metal hydrides, have recently received considerable attention for potential use in heat and energy conservation schemes. It should be understood that "hydrides" refers generally to hydride-forming materials in both the hydrided and dehydrided states.
Typically, each hydride has an equilibrium temperature which is a function of hydrogen pressure. When the hydride is raised to a temperature greater than the equilibrium temperature and heat is further supplied, the hydride will begin to decompose, giving off previously absorbed hydrogen. Conversely, the hydride will absorb hydrogen when its temperature is lowered and maintained below the characteristic equilibrium temperature of the prevailing hydrogen pressure. Thus, the quantity of hydrogen stored in the hydride can be varied by adjusting the temperature and hydrogen partial pressure in contact with a hydride and supplying or removing heat as appropriate. The materials of primary interest in this context are those which are exothermic absorbers of hydrogen. For these, appreciable quantities of heat are released as the material absorbs hydrogen.
Hydrides that have found most frequent application in the developing fields of heat and energy conservation are predominantly pure metals, metal alloys, or intermetallic compounds which are capable of storing large quantities of hydrogen in the metallic lattice. The amount of hydrogen stored in the lattice can be expressed as the atom ratio H/m, where H is the number of hydrogen atoms and m is the number of metal atoms. A useful characteristic of many hydrides is that the equilibrium pressure for a given temperature is approximately constant for a wide range of absorbed hydrogen concentration during the hydriding or dehydriding cycle. In terms of the atom ratio, this means that graphs of equilibrium pressure versus H/m at given temperature (pressure-composition isotherms) exhibit a nearly flat isobaric "plateau" region over which large quantities of hydrogen may be absorbed/desorbed while the pressure is kept relatively constant.
The equilibrium temperature for a given pressure is not the same for all hydride-forming materials. On the contrary, different hydrides exhibit a wide range of equilibrium temperatures for a given pressure. It is this variation that permits the use of two or more hydrides in combination in heat pumping schemes, for example. For some of the more useful hydrides, higher equilibrium temperatures are generally associated with higher hydrogen pressures.
One of the problems encountered in the use of hydrides, particularly hydrides of metal alloys and intermetallic compounds, is the phenomenon of disproportionation--the degradation or loss of effective hydrogen absorbing capacity of a hydride after a number of absorption/desorption cycles.
Disproportionation is characterized by a reduction in the quantity of hydrogen a given hydride is capable of absorbing (capacity) and a deterioration of the pressure-composition "plateau" characteristics. Hydrides that are preferred in most applications are those that are capable of attaining large values of H/m (display appreciable absorption of hydrogen), and possess pressure-composition isotherms that are substantially isobaric over a large range of H/m values (with large, flat plateau regions). As disproportionation begins to occur, however, the hydride begins to deviate significantly from this isobaric behavior. In practical terms, this generally means that, for a given temperature, the hydrogen pressure needs to be continually increased to maintain continued hydrogen absorption. The concomitant result is loss of efficiency, less favorable kinetics, and in most applications, diminished usefulness of the particular disproportionated hydride.
The interest in hydrides for heat and energy conservation applications has only recently resulted in a recognition of the problem of disproportionation. The literature references to disproportionation are therefore scarce at this time. A paper by F. E. Lynch and E. Snape, "The Role of Metal Hydrides in Hydrogen Storage and Utilization", given at the 2nd World Hydrogen Energy Conference in Zurich, Switzerland, Aug. 21, 1978, cites disproportionation as a major problem yet to be solved.
Other researchers are becoming aware of the problem through empirical observation. In particular, Buschow and Miedema, in a paper entitled "Hydrogen Absorption in Rare Earth Intermetallic Compounds" (delivered at the Hydrides for Energy Storage International Symposium held in Geilo, Norway, Aug. 14-19, 1977, page 235) noted that repeated cycling of ternary hydrides may result in decomposition and loss of capacity. Buschow et al. proposed that regeneration could be achieved by annealing the material in vacuum (page 246). In making this statement, Buschow et al. cited a peper by Cohen et al. ("Degradation of Hydrogen-Absorbing Rare Earth Intermetallics by Cycling," Solid State Commun., 1978, 25 (5), 293) which provides a detailed treatment of the degenerative effects of repeated cycling. Cohen et al. also observe that original H.sub.2 absorbing capacity can be restored to EuRh.sub.2 by annealing it at 350.degree. C. for 5 hours under vacuum.
Yamanaka et al. ("Hydride Formation of Intermetallic Compounds of Titanium-Iron, Titanium-Cobalt, Titanium-Nickel, and Titanium-Copper," J. Chem. Soc. Japan, 1975, No. 8, 1267-1272) noted that cycling of a hydride of the Ti-Cu system resulted in a compound that displayed x-ray diffraction lines due to TiH.sub.2 and copper only--the lines for TiCu or Ti.sub.2 Cu had disappeared. Degassing the compound at about 850.degree. C. resulted in a return to the original Ti-Cu intermetallic compound.
While researchers are becoming aware of disproportionation and its effects, the art has thus far failed to determine both the definitive general cause of the phenomenon or a general economical solution to its degenerative effects on a hydride. Thus far, one suggested general "solution" to the problem has been to raise the hydride-former to a temperature above its melting point for some length of time. This, of course, is really no solution at all, but rather a remelting and recasting of the hydride-former itself--simply a refabrication of the desired hydride-former from its constituents. This approach is both time consuming and expensive. In addition, reducing the hydride-former to a non-particulate or molten state also introduces handling problems and limitations on where the hydride can be regenerated. It should be noted that all references to the melting point of a hydride are intended to refer to the melting point of the dehydrided (hydrogen-free) hydride-forming material. This distinction is necessary due to the fact that the melting point of a hydrided (hydrogen-rich) material may differ from the melting point of the corresponding dehydrided hydride-former.
In certain specific cases, attempts to regenerate a specific hydride species have involved annealing the hydride at very low or vacuum pressures. The annealing temperature range for most alloys is generally considered to be greater than one-half, and preferably greater than two-thirds, the Kelvin melting point of the alloy. Typically, such a process can involve temperatures in excess of about 700.degree. C., and commonly, in excess of 900.degree. C. Attaining and maintaining such temperatures while simultaneously maintaining vacuum pressures over the hydride bed can be difficult and expensive.
It has been hypothesized that disproportionation is due to the formation of a hydride state which is more stable than the desired hydride state. That is, for a given hydride, there may exist more than one chemical reaction by which hydrogen may be absorbed. While one chemical reaction may form the hydride which possesses desirable pressure-composition characteristics, there may also exist a more thermodynamically favored reaction which forms a hydride with markedly different, and less favorable, characteristics.
For clarity, the postulated disproportionation process is described with reference to a particular alloy, lanthanum-nickel, although it is to be understood that the general descriptions and statements are equally applicable to other metallic and some non-metallic hydrides as well. In the dehydrided (or substantially hydrogen-free) state, the most thermodynamically favored configuration for a useful lanthanum-nickel alloy is LaNi.sub.5. The hydride of LaNi.sub.5 possesses isotherms with desirable absorption/desorption characteristics (i.e. large capacity and isobaric behavior for a large range of H/m values). The desired hydriding reaction is EQU LaNi.sub.5 +3H.sub.2 .fwdarw.LaNi.sub.5 H.sub.6 +heat.
It should also be noted, however, that the formation of elemental lanthanum hydrides or lower nickel content lanthanum-nickel hydrides are thermodynamically more favored reactions in the presence of hydrogen.
There has been considerable speculation among workers in the field concerning the cause of disproportionation. One of the best explanations is that disproportionation occurs as the LaNi.sub.5 becomes substantially hydrided (hydrogen rich), and diffusion effects in the lattice cause a highly localized migration or separation of the La and Ni atoms--thus providing a mechanism for the formation of the lower nickel content lanthanum hydride compounds. While this theory is gaining acceptance, the invention described herein is not dependent upon its accuracy and should not be limited thereby.
In theory, as hydrogen enters the lattice, the lattice expands and the hydrogen present makes it thermodynamically more favorable for the nickel and lanthanum to migrate apart (perhaps a distance as small as one or two lattice sites). This effect can be more pronounced at the surface of the hydride. In a very localized region of the lattice, therefore, one would find segregation of the lanthanum and nickel atoms. As segregation begins to occur, lower nickel content lanthanum hydrides begin to form because there is less nickel in the lattice sites immediately adjacent to a given lanthanum atom. This microscopic "diffusion-segregation-low nickel hydride formation" process may begin at the first entry of hydrogen into the lattice and may continue throughout the absorption process. Further, the lower nickel content hydrides are forming simultaneously with the desired hydride, LaNi.sub.5 H.sub.6. Consequently, as the absorption process continues, there will exist a number of hydride species within the given sample of material. This microscopic aggregate of different species would, of course, be expected to exhibit macroscopic pressure-composition characteristics quite different from those expected of the desired species alone.
Some metallic hydrides, magnesium-nickel for example, appear to resist disproportionation at almost all temperatures. In the context of the prevailing theory, such exceptions are explained by the speculation that the desired hydrogen-free alloy remains thermodynamically favored even in the presence of hydrogen. Identification of which hydrides disproportionate is accomplished by routine experimentation.
The diffusion and migration of the distinct atoms within the lattice appears to occur to some degree at all temperatures. The rate of diffusion is, however, observed to increase rapidly with increasing temperature. The disproportionation reaction therefore appears to be a thermally activated phenomenon, (accelerating with increasing temperature) providing hydrogen is present to cause the thermodynamics to favor the formation of alternative hydrides. Frequently, the higher the temperature at which a given hydride absorbs hydrogen, the greater the disproportionation occurring during the absorption, and the larger the corresponding degeneration in absorption plateau characteristics.
The alteration of the pressure-composition characteristics of hydrides poses great problems in most applications. Such degradation of capacity in any process application would necessitate continual removal and replacement of the hydride. Any type of application in which the pressure-composition characteristics are expected to be reproducible over a number of cycles would suffer from the gradual change in hydride properties during sustained usage.
There is therefore a need for a new method of reproportionating hydrides so that the original characteristics of a disproportionated hydride are substantially restored. In particular, there is a need for a solution to the disproportionation problem that is easier, less expensive, less time consuming and requires less specialized equipment than the current art solution of either literally melting the hydride down and re-fabricating it from its constituents or annealing the hydride at high temperatures and vacuum pressures. Moreover, for anticipated commercial applications of hydrides, there is a need for an on-line or in-situ method of regenerating hydrides.
The general object of this invention is to provide a method of reproportionating a hydride to substantially restore the hydride's original pressure-composition characteristics. Another object of this invention is to provide a method of reproportionating a hydride at a temperature below the melting and annealing temperatures of the hydride. Another object of this invention is to reproportionate a hydride under conditions less severe in pressure, temperature and duration than those required to originally fabricate or anneal the hydride. A further object of this invention is to provide a method of reproportionating a hydride without reducing the hydride to a molten or non-particulate state. Still another object of this invention is to provide a method of reproportionating a hydride which can be practiced in-situ--without the need to interrupt the operation of the process in which the hydride is being used. Still other objects of this invention will become apparent to those skilled in the art after consideration of the drawings and following descriptions.